1. Field of the Invention
The present invention relates to a rubber composition composed of a block copolymer comprising two different polymer block units and having polymer terminals modified with an isocyanate compound, said rubber composition using a conjugated diolefine and a monovinyl aromatic hydrocarbon as starting materials.
2. Related Art statement
Recently, in order to improve fuel mileage and running safety of automobiles, materials having a smaller rolling friction resistance, a larger wet skid resistance and a larger ice skid resistance have been desired as rubbers for use in tire treads of the automobiles.
However, the above resistances are contradictory to one another. Further, when these resistances as well as the wearing property cannot be simultaneously satisfied by a single rubber. Under the circumstances, blended compositions of different kinds of rubbers have been used to harmonize the above properties. For instance, there has been used, as a rubber for the tire treads of the automobiles, a blended composition consisting of a styrene-butadiene copolymer rubber having a relatively large wet skid resistance and containing 10 to 30% by weight of a bound styrene and not more than 20% by weight of vinyl bounds and a polybutadiene rubber having a small rolling friction resistance and a good wear resistance and containing not more than 20% of vinyl bonds. However, such a blended composition does not reach a satisfied level particularly with respect to the wet skid resistance.
In recent years, SBR (styrene-butadiene rubber) and BR (butadiene rubber) obtained by mainly using an organic lithium initiator has been attempted to be modified so as to improve the wet skid resistance and the rolling friction resistance. In this connection, a rubber composition containing BR with much vinyl bonds has been lately proposed as a rubber composition meeting the above object (see British patent No. 1166832). Although the BR containing much vinyl bonds is excellent in balance between the wet skid resistance and the rolling friction resistance, it is extremely poor in the fracture property and the wearing property. Therefore, it is difficult to singly use such a BR as a rubber. Thus, in order to improve the fracture property and the wearing property, a blended composition of BR and a diene rubber such as NR (natural rubber), high cis-BR or an emulsion polymerized SBR has been proposed. However, the blended composition of NR or high cis-BR and BR has the shortcoming that it is inferior in the balance among the wet skid resistance, fracture property and the wearing property. On the other hand, the blended composition of BR and SBR is unfavorably inferior in the rolling friction resistance.
Further, according to Japanese patent application Laid-open No. 54-62,248, the wet skid resistance and the rolling friction resistance are attempted to be improved by using random SBR containing 20 to 40% by weight of bound styrene and having a relatively high amount of vinyl bonds. As compared with the conventional emulsion polymerized SBR and SBR obtained by using an organic lithium initiator, balance among the wet skid resistance, the rolling friction resistance and the wearing property are indeed improved, but it is still unsatisfactory for practical application. Thus, it must be used in a form blended with another diene rubber. However, as in the case of the high vinyl content BR, only insufficient balance among the wet skid resistance, rolling friction resistance, the fracture property, and the wearing property have not been obtained for a blended system with any rubber.
In view of the above situation, the present inventors have strenuously made effort to obtain a rubber material having an excellent balance among the wet skid resistance, the rolling friction resistance and the wearing property, and as disclosed in Japanese patent application Nos. 55-178,226, 55-186,194 and 55-186,195, they found that the balance among the above properties can be improved by selecting the content of bound styrene and the content of vinyl bonds of block portions in specified ranges with respect to a styrene-butadiene block copolymer composed of random copolymer blocks each having different bound styrene content and vinyl bond content. However, since social demand has increasingly become higher with respect to the safety and low fuel mileage for the tires, the tires obtained from a single use of the block copolymer or a blend of a different kinds of a rubber therewith still have a room to be improved in respect to the wearing property and the fracture property.
As disclosed in Japanese patent application No. 56-85,481, the present inventors have found after further strenuously improving effort that a rubber composition containing a block copolymer having a Mooney viscosity (ML (100.degree. C., 1+4)) of 20 to 150, composed of two kinds of specific random copolymer blocks having different contents of monovinyl aromatic hydrocarbon, and having metal (particularly preferably tin) and carbon bonds in molecular chains of a part of the polymer meets the above object.
However, since polymer terminals in the thus obtained rubber composition are carbon-tin bonds, the carbon-tin bonds at the polymer terminals are easily cut due to a chemical reaction between polymer terminals and an acidic substance such as an inorganic acid, an organic carboxylic acid and a Lewis acid, an organic phosphorus compound, or an organic sulfur compound, or hydrolysis in the presence of a strong acid or a strong alkali, the molecular weight largely varies and the rubber composition is therefore unstable. When the molecules are cut in such a way, rebound resilience and tensile strength are extremely lowered. Accordingly, the polymer has restrictions upon kinds of a polymerization terminator and an antioxidant, desolvation conditions, a kind of an oil extender to be added, a kind of other polymer to be blended, etc. in the production of the polymer.